Bring a large pot of water to boil. Add potatoes, lower heat to a simmer, and cook until very tender, about 25 minutes. Drain potatoes and return to pot over low heat. Stir over heat until completely dry and starting to fall apart (this will make them soft and light when puréed).
Meanwhile, heat cream and milk in a heavy-bottomed pot over medium-low heat until just simmering. Put potatoes, butter, and horseradish into bowl of a standing mixer with paddle attachment and beat on low until nearly smooth. While beating, drizzle enough cream-milk mixture into potatoes to make them soft and loose, keeping in mind that potatoes stiffen as they sit around (you may not need all the milk and cream). Add salt and white pepper to taste and beat just until smooth (avoid overbeating, as purée will become gummy).
Note: Nutritional analysis is per serving.